Thanks Bob,
I'm fairly new to building engines and I know that the lifters can't be compressed when full. BUT They don't bleed down in a couple of cycles, they stay solid and keep the valve open AT ALL TIMES. Even when the engine is running. I got virtually 0 compression on all cylinders even after a few 100 miles of driving, because the valves are always slightly open. And I know what preload to set them at, and that preload has to be set with the lifters empty. But that doesn't change my problem that the lifters don't bleed down enough. Maybe I should try getting stronger springs.

But let's suppose you are completely right. I have these symptoms:
-Engine works well
-drive 300 mls
-engine loses power, gets hot uses lots of fuel
-after 500 mls engine runs very rough, so rough that the tranny doesn't shift anymore, hard starting,...
-disassemble engine, clean lifters, put it back together again (don't change anything else)
-engine runs fine for 300 mls...

What else could that be? Can it have something to do with wrong viscosity oil?